To control industrial processes and systems, e.g., offset printing machines and machine tools, it is often necessary to control, i.e., drive, a plurality of individual systems in synchronization. In a printing system, for example, the axes of individual printing stations must be controlled very precisely over time to ensure satisfactory printing of the print product. In the case of a machine tool, individual axes must be triggerable in a mutually coordinated manner with high time precision.
Control of such a complex industrial system is usually implemented by a decentralized distributed control system. In this way, through the use of modular electronic controls, a plurality of different control systems may be composed of a few basic components. Modular control and regulating circuits are typically used for individual components of the system. These control and regulating circuits are interconnected via a field bus which permits mutual data exchange and connection to a control console.
To ensure a precise interaction of the individual system components over time, a common time base must be supplied in all control and regulating circuits. Only in this way is it possible to ensure the required coordination among the individual decentralized control circuits.
It is customary in printing machine control to supply synchronization signals corresponding to a virtual axis signal to individual drives, for example. By analogy with a mechanical drive, a main control unit generates a signal corresponding to a virtual longitudinal wave. Slave units generate a virtual standing wave signal starting from each longitudinal wave signal.
DE-A-199 17 354 describes a synchronization method for synchronizing a master unit with slave units. The master unit and the slave units each have their own timers. The timers of the slave units are synchronized with the timer of the master unit at regular intervals to minimize deviations between the timers. To do so, the master unit transmits signals containing the transmission time to a slave unit over two ring-shaped communication paths. These time signals require different transit times to the slave unit. The slave unit measures the difference between the signal transit times and receives from the master unit a cycle time for the signal throughput through the ring-shaped communication paths. The slave unit calculates from this the transit time of the time signals from the master unit over the particular communication path. The time information sent by the master unit is corrected using the calculated transit time to determine the expected reception time. The timer of the slave unit may be corrected based on the comparison of the expected reception time with the reception time measured by the slave unit.
One disadvantage of this traditional method is that after ascertaining the transit time difference, each slave unit must calculate the individual signal transit time of the individual communication paths. In addition, each slave unit must also determine which signal path the time signal arrives on first to ensure a correct calculation of the transit times. Furthermore, the method proposed in the publication in question is based on each slave unit and the master unit having individual timers. Because of the limited accuracy of such timers, a deviation among the individual timers may develop within a short period of time and may be substantial in comparison with the required timing precision. Therefore, despite the use of individual timers, the individual units must be resynchronized frequently.
The IEEE1588 standard entitled “Precision Clock Synchronization Protocol for Networked Measurement and Control Systems” (abbreviated PTP) is often used for cyclic synchronization of individual timers. This standard defines a method for synchronizing multiple real time clocks distributed in space and connected via a network such as Ethernet. According to PTP, one station is established as the master clock and sends the other stations a first sync telegram and then a follow-up telegram specifying the exact point in time of the first telegram. The receiving station is able to calculate the time difference between its clock and the master clock on the basis of the first telegram, the follow-up telegram and its own clock and thereby perform a clock synchronization.
A machine control system is described in US 2002/0110155. This machine controller has a central control unit and a plurality of secondary nodes, each controlling one actuator. Each node stores a delay parameter for signals from the central control unit. The particular delay parameter depends on the signal transit time between the central control unit and the node. The central control unit sends time messages which trigger a synchronized implementation of control commands in the nodes in cooperation with the delay parameters. To determine the signal transit time from the central control unit to each node, the control unit sends a data telegram to each node which then sends it back. The central control unit calculates the transit time delay of the signal up to the node from the period of time until the signal returns and sends this information to the node.
One disadvantage of this method is that it requires an extensive initialization phase during which the central control unit determines the signal transit time to each node and sends this information to the individual nodes.